Why does `cd` have no effect if output is piped?

Question

I accidentally typed

cd /tmp | tail

which is of course not what I meant. But to my surprise, I then did not change directory. Why is this? For comparison

cd /tmp > /tmp/foo

does result in a change of directory, so it's not the output redirection per sé. What's going on here?

Answer 1

Pipes involve subshells, portions of which are not run in the parent shell and thus cannot affect the working directory of that parent shell process. This is how ZSH behaves:

% cd /tmp
% cd /var/tmp | tail
% pwd
/tmp
% echo foo | cd /var/tmp
(pwd now: /var/tmp)
% cd /tmp
% echo foo | cd /var/tmp | echo bar
bar
% pwd
/tmp
% 

Note how the directory only changed when the cd was the last command in the pipeline; this was run in the parent shell proper and thus was able to change the working directory of that process.

A useful use of this feature usually involves an explicit subshell and commands run therein:

dowork | ( cd elsewhere && domorework ) | andyetmore

also be sure to error check the cd call instead of assuming it worked, unless you like rsync output sprayed all over /, or other such hypothetical messes...